The use of optical disk media, for example, a DVD-RAM and a DVD-RW made of a rewritable phase-change optical write material has been widely distributed. Recently a high-capacity Blu-ray Disc conforming to a recording of a high-vision broadcasting program has been commercialized using blue laser. Commercialization of the optical disk device which allows a single drive unit to write and read the information not only on the DVD but also on the Blu-ray disk is further demanded. In the case where the information data are written on the thus high density optical disk, it is essential to optimize the write laser beam power and pulse conditions, in other words, to perform so called “trial writing”.
Generally quality evaluation with respect to the signal written on the optical disk has been performed using a jitter as a standard deviation of the difference between the data edge and the clock edge. A specific measurement instrument such as a jitter analyzer is required to measure the jitter value. As the jitter analyzer is too expensive to be installed in the optical disk device, an alternative indicator for the signal evaluation has been demanded.
In JP 1998-320777 A (corresponding to U.S. Pat. No. 6,269,062), the jitter value is not directly measured but evaluated equivalently by counting the number of logical pulses referred to as the error pulse generated in response to the moment when the phase difference between the data edge and the clock edge becomes equal to or greater than a predetermined value. This makes it possible to optimize the write power.
International Publication No. WO01/011614 (corresponding to U.S. Pat. No. 6,654,325) discloses the technique for trial writing on the DVD-RAM with capacity of 4.7 GB under the adaptive write pulse/power conditions (write strategy) of table reference type in accordance with the previous and subsequent space lengths and mark lengths. The error pulses are sorted in reference with the table of the write strategy for optimizing the pulse conditions of the write laser beam such that the error pulse value of the respective sections of the table is minimized. The actual sorting requires two 4×4 tables; however, because the logical pulse, that is, error pulse is used for the signal evaluation, the aforementioned sorting may be easily performed with the logical LSI.
In JP 2003-6864 A, the process based on binarization with a generally employed direct slice process as described above is further improved to perform optimization of the write conditions conforming to the binarization through Viterbi decoding. In the aforementioned process, the readout signals that have been A/D converted are sorted in accordance with the historical transition of the target signal level for Viterbi decoding. Then the phase differences between the clocks and data edges are detected, based on which the write condition optimum for Viterbi decoding is obtained.